In recent years, there have been demands for developments in recording media for storing information recorded/processed, in view of portable information recording/processing terminals, such as cellular phones, PDAs (Personal Digital Assistants), mobile personal computers, video cameras, digital still cameras, portable recording/reproducing devices.
Developed as those recording media have been devices using optical discs as removable media which are cheap and easy to handle. Studies have further been being made on these devices.
Portable information recording/processing terminals have a compact size from the viewpoint of conveniences, so the optical discs to be used must have a small size. However, the volume of data recordable in every disc must be large in consideration of, for example, use for video cameras and PDAs which deal with data of large information amounts.
In order to realize recording or reproducing data at a high recording density, there has been a popularly used method. In this method, the wavelength of a recording or reproducing laser beam has to be shortened, and the numerical aperture (NA) of an objective lens which converges the recording or reproducing laser beam onto an optical disc has to be enlarged. Based on this method, discussions are made on a structure, for example, using blue light having a wavelength of 450 nm or less for the recording or reproducing laser beam, setting the NA of the objective lens to 0.78 or more, and an optical disc having a thinner protection layer than 0.1 mm.
If the optical disc having this thinner protection layer is applied to an optical disc of a type having a small diameter of about 50 mm, for example, data whose volume is as large as 2 GB or more can be recorded and reproduced per disc.
Meanwhile, optical discs used for portable information recording/processing terminals are exposed to extreme changes in temperature and humidity due to environmental changes as the discs are carried. Hence, warping of the discs due to changes in temperature and humidity must be small with respect to the optical discs used for portable information recording/processing terminals.
In case of the small diameter optical discs having a thin protection layer as described above, detailed knowledge has not yet been stocked about the warping according to changes in temperature and humidity. There hence has been a conventional way of thinking that, for example, the warping due to changes in temperature and/or humidity need not be considered but the disc thickness may be decided fixing the ratio between the disc diameter and the disc thickness.
If the disc thickness is thus decided without considering changes in temperature or humidity, the optical discs are warped greatly due to changes in temperature and humidity. This leads to a problem that accurate recording/reproducing of information signals cannot be performed on the optical discs.